The present invention relates to a system for controlling the output power of a motor vehicle such as an automobile, for example.
Road conditions abruptly vary from time to time while a motor vehicle is running, or a motor vehicle often runs on slippery road surfaces with low coefficients of friction, such as snow- or ice-covered roads. In such cases, the drive wheels of the motor vehicle are apt to spin, making it difficult for the driver to control the motor vehicle.
One practical way of getting the motor vehicle under control in such conditions is for the driver to adjust the depression of the accelerator pedal for controlling the output power of the engine so that the drive wheels do not spin. However, even highly skilled drivers find it difficult to make fine accelerator pedal adjustments while driving.
While a motor vehicle is making a turn, the motor vehicle is subjected to a centrifugal force corresponding to the lateral acceleration which acts in a direction perpendicular to the direction of travel of the motor vehicle. If the speed of the motor vehicle is too high with respect to the turning circle, then the motor vehicle will slip laterally under the centrifugal force that exceeds the gripping force of the tires.
In order to drive the motor vehicle safely with a turning radius corresponding to the turning circle while lowering the output power of the engine, the driver is required to be highly adept at driving particularly if the end of the turning circle cannot be confirmed or the radius of curvature of the turning circle is progressively smaller.
When a general motor vehicle with an understeer response makes a turn, the steering angle of the steered wheels has to be progressively increased as the lateral acceleration on the motor vehicle increases. If the lateral acceleration exceeds a certain value inherent in the motor vehicle, the steering angle is greatly increased, making it difficult or impossible to turn the motor vehicle safely. As is well known in the art, the above tendency is greater with front-engine front-drive motor vehicles which have a stronger understeer response.
There has been proposed a motor vehicle output power control system for detecting a spinning condition of a drive wheel of a motor vehicle and forcibly lowering the output power of the engine on the motor vehicle irrespective of the depression by the driver of the accelerator pedal if the driven wheel spins, or for detecting a lateral acceleration of a motor vehicle and forcibly lowering the output power of the engine on the motor vehicle irrespective of the depression by the driver of the accelerator pedal before a limit condition as indicated by the lateral acceleration, in which it is difficult or impossible to turn the motor vehicle safely, is reached. The driver can select a control mode in which the motor vehicle output control mode is in operation to control the output power of the engine, or a normal mode in which the output power of the engine is controlled solely depending on how deeply the accelerator pedal is depressed.
According to known principles of such a motor vehicle output control system, the rotational speeds of drive and driven wheels are detected with the difference between the detected rotational speeds being regarded as a slip of the drive wheel, and the drive torque produced by the engine is controlled depending on the slip, or the yaw rate of the motor vehicle is detected, and the drive torque produced by the engine is controlled depending on the yaw rate.
The yawing of the motor vehicle while it makes a sharp turn during high-speed travel becomes greater as the vehicle speed is higher and the turn is sharper. When the yaw rate of the motor vehicle is detected by a vibration sensor, an acceleration sensor, or the like, or when the yaw rate goes higher than a predetermined value, the drive torque of the engine is reduced.
For greater safety during a turn, a target drive torque may be established on the basis of a lateral acceleration which is detected, and the drive torque of the engine may be controlled to reach the target drive torque, irrespective of the depression of the accelerator pedal.
With the above motor vehicle turn control systems, since the drive torque is controlled on the basis of only detected conditions of the motor vehicle, the drivability of the motor vehicle may not be fully achieved. For example, even if the driver wants to accelerate the motor vehicle at the end of a turning circle, because the drive torque has been determined by the control system irrespective of the depression of the accelerator pedal, the driver's intention is not reflected.
It is not practical to entirely ignore the drive torque which is demanded by the driver during operation of the control system. In view of this, it may be possible to determine the drive torque demanded by the driver from the rotational speed of the engine and the accelerator opening. If the demanded drive torque is smaller than the target drive torque, then the demanded drive torque may be employed to control the engine, and if the demanded drive torque is larger than the target drive torque, then the target drive torque may be corrected according to the demanded drive torque.
However, the output signal from an accelerator opening sensor may not accurately represent the accelerator opening. The output signal from the accelerator opening sensor and the actual accelerator opening are proportional to each other, and the accelerator opening sensor is installed such that its output signals are constant when the accelerator is fully closed. When the accelerator opening sensor is detached and attached again for motor vehicle inspection, it is troublesome and virtually impossible to put the accelerator opening sensor exactly back into its original position. In addition, the accelerator opening sensor may be displaced in position over a long period of time.
Since the output signal from the accelerator opening sensor may not accurately represent the actual accelerator opening for the above reasons, the fully closed position of the accelerator should be detected from time to time in some way, and the detected accelerator opening should be corrected according to the fully closed position.